A structural engineer is designing a concrete foundation for a commercial building in a region with highly expansive soils. To mitigate potential damage from soil movement, an isolated spread footing is being considered. Which of the following strategies is most appropriate for designing the footing to resist uplift forces caused by soil expansion?
- AIncrease the depth of the footing to place the bearing surface below the active expansive zone.
- BDecrease the width of the footing to reduce the surface area exposed to expansive soil.
- CIncrease the reinforcing bar size and number in the footing.
- DReduce the concrete cover over the reinforcement to allow for greater flexural deformation.
Show answer & explanationAnswer & explanation
Correct answer: A. Increase the depth of the footing to place the bearing surface below the active expansive zone.
For foundations in expansive soils, the most effective strategy to resist uplift is to place the footing's bearing surface below the depth of seasonal moisture variation, known as the active expansive zone. This minimizes the soil's ability to exert uplift pressure on the footing. Increasing the depth also increases the dead load of the soil above the footing, which provides additional resistance to uplift.
Why the other options are wrong
- B. Decreasing the width reduces bearing area, which could lead to increased bearing pressure and settlement, and while it reduces exposed area, it also reduces the footing's self-weight contribution to resisting uplift.
- C. While important for strength, increasing rebar alone doesn't prevent uplift from expansive soils effectively; it deals with internal stresses, not the external force.
- D. Reducing concrete cover compromises durability and structural integrity, and does not address the uplift force from expansive soils.
Foundations in Expansive Soils
Design considerations for foundations built on soils that significantly change volume with moisture content, primarily to resist uplift and settlement.
- Expansive soils cause uplift when wet and settlement when dry.
- Deep foundations or careful moisture control are common solutions.
- Understanding the 'active zone' depth is crucial.
Memory trick: Expansive soil is like a 'Spring' that pushes up; 'Deep' foundations 'Defy' its reach.